The use of add-on heat pump systems is becoming popular. The use is in the area where retrofit applications of heat pumps replace faulty air conditioning units with the heat pump supplying cooling during the summer and a first stage of heating during the winter. The second stage of heating use the existing fossil fuel type of furnace. This system would operate in a cost attractive manner in geographical locations where the ratio of electricity cost to fossil fuel cost is relatively low. Operating the heat pump with a high coefficient of performance during mild weather means a lower heating bill during those times.
With an add-on heat pump system, the indoor coil is normally located in a warm air furnace plenum. This means that the fossil fuel furnace must be turned off when the heat pump is operating. The failure to turn off the heat pump when the furnace is operating can cause severe damage to the heat pump equipment located downstream of the warm air furnace as the heat from the warm air furnace combined with the heat from the heat pump exceeds the design limits for the heat pump coil.
Today there are two basic strategies for controlling these add-on heat pump type systems. An outside thermostat setpoint dictates which system should be operational from the space thermostat. The heat pump is operated above the setpoint and the fossil fuel furnace operated below the setpoint. The setpoint of the outdoor thermostat in this case would be adjusted to represent the balance point of the heating requirements of the house and output capacity of the heat pump. Also, these types of add-on heat pump systems have been operated with two-stage thermostats used in conjunction with system logic controls (relay panels, duct thermostats, etc.) to achieve the proper sequence of operation between the heat pump and the fossil fuel furnace.
Because of the varying internal loads, the variations in outdoor temperature, the variation in solar effects, etc. the balance point of a house is continually changing. Therefore, an outdoor thermostat controlling changeover must be set for the worst case, and the actual saving is less than optimum. The conventional two-stage thermostat control approach requires additional logic to test whether the heat pump has the capacity to heat the house during cool outdoor weather. Without this type of test, once the thermostat starts to operate the second stage fossil fuel furnace it would continue to cycle the furnace without reverting back to the heat pump when outdoor temperatures would allow this type of operation in a more economical manner. A drawback of the conventional two-stage thermostat for this application is that during normal morning pickup, the second stage would be initially energized and the fossil fuel furnace would operate to pick up the space even though in warm weather that might not be necessary. Therefore, the more efficient heat pump would not be utilized.